Measurement of optical characteristics of optical fibers has been carried out conventionally. For example, Patent Document 1 (Japanese Laid-Open Patent Publication (Kokai) No. H9-264814) describes a polarization mode dispersion measuring device for optical fibers. With reference to FIG. 4, a description will now be given of the device for measuring the polarization mode dispersion of optical fibers according to Patent Document 1.
First, the polarization mode dispersion τPMD of an optical fiber under test 104 is defined by the following equation (1):
[EQU. 1]τPMD=2√{square root over ({dot over (θ)}2+{dot over (ψ)}12 cos2θ+{dot over (ψ)}22 sin2θ)}  (1)
It should be noted that, in the above equation, θ denotes a polarization angle, ψ1 denotes a phase shift in a certain direction on a plane perpendicular to the propagation direction of light, and ψ2 denotes a phase shift in a direction orthogonal to ψ1. On this occasion, a transfer function matrix [T] of the optical fiber under test 104 is defined by the following equation (2):
[EQU. 2]
                              [                      T            ⁡                          (              ω              )                                ]                =                  [                                                                                                                                    T                      11                                                                            ⁢                                      ⅇ                                          -                                              jϕ                        11                                                                                                                                                                                                        T                      12                                                                            ⁢                                      ⅇ                                          -                                              jϕ                        12                                                                                                                                                                                                                                T                      21                                                                            ⁢                                      ⅇ                                          -                                              jϕ                        21                                                                                                                                                                                                        T                      22                                                                            ⁢                                      ⅇ                                          -                                              jϕ                        22                                                                                                                          ]                                    (        2        )            
It should be noted that, in the above equation, |Tij| denotes an amplitude of respective matrix elements, φij denotes a phase shift of the respective matrix elements, and both of them are functions of an optical angular frequency ω. Thus, the parameters θ, ψ1, and ψ2 in the equation (1) are respectively obtained by the following equations (3), (4), and (5).θ(ω)=0.5 cos−1(|T11|2−|T21|2)  (3)ψ1(ω)=(φ11−φ22)/2  (4)ψ2(ω)=(φ21−φ12+π)/2  (5)
Consequently, the polarization mode dispersions τPMD of the optical fiber under test 104 is obtained by obtaining the transfer function matrix [T] of the optical fiber under test 104.
A description will now be given of how to obtain the transfer function matrix [T] of the optical fiber under test 104 with reference to FIG. 4. First, a control section 109 makes output light of a polarization controller 103 as a linearly polarized wave coincident with a p-direction of a polarization beam splitter 105 incident to the optical fiber under test 104. On this occasion, output light from the optical fiber under test 104 is represented by the following equation (6):
[EQU. 3]
                                          [                                                                                                                                                    T                        11                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          11                                                                                                                                                                                                                              T                        12                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          12                                                                                                                                                                                                                                                          T                        21                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          21                                                                                                                                                                                                                              T                        22                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          22                                                                                                                                          ]                    ⁡                      [                                                            1                                                                              0                                                      ]                          =                  [                                                                                                                                    T                      11                                                                            ⁢                                                            ⅇ                                              -                                                  jϕ                          11                                                                                      ⁡                                          (                                              p                        ⁢                                                                                                  ⁢                        component                                            )                                                                                                                                                                                                          T                      21                                                                            ⁢                                                            ⅇ                                              -                                                  jϕ                          21                                                                                      ⁡                                          (                                              s                        ⁢                                                                                                  ⁢                        component                                            )                                                                                                    ]                                    (        6        )            
The above-described output light is split into an s-polarization component and a p-polarization component by the polarization beam splitter 105, and then, the components are made incident to O/E converters 1061 and 1062 respectively, and the O/E converters 1061 and 1062 respectively measure:
[EQU. 4]|T11|e−jφ11,|T21|e−jφ21
After the above-described measurement, the control section 109 rotates output light of the polarization controller 103 at 90°, and makes the resulting light as a linearly polarized wave coincident with an s direction of the polarization beam splitter 105 incident to the optical fiber under test 104. On this occasion, output light from the optical fiber under test 104 is represented by the following equation (7):
[EQU. 5]
                                          [                                                                                                                                                    T                        11                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          11                                                                                                                                                                                                                              T                        12                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          12                                                                                                                                                                                                                                                          T                        21                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          21                                                                                                                                                                                                                              T                        22                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          22                                                                                                                                          ]                    ⁡                      [                                                            0                                                                              1                                                      ]                          =                  [                                                                                                                                    T                      12                                                                            ⁢                                                            ⅇ                                              -                                                  jϕ                          12                                                                                      ⁡                                          (                                              p                        ⁢                                                                                                  ⁢                        component                                            )                                                                                                                                                                                                          T                      22                                                                            ⁢                                                            ⅇ                                              -                                                  jϕ                          22                                                                                      ⁡                                          (                                              s                        ⁢                                                                                                  ⁢                        component                                            )                                                                                                    ]                                    (        7        )            
The above-described output light is split into an s-polarization component and a p-polarization component by the polarization beam splitter 105, the components are made incident to the O/E converters 1061 and 1062 respectively, and the O/E converters 1061 and 1062 respectively measure:
[EQU. 6]|T12|e−jφ12,|T22|e−jφ22
A network analyzer 107 obtains the parameters θ, ψ1, and ψ2 from the respective parameters measured as described above, and the equations (3), (4), and (5). It should be noted that the network analyzer 107 controls an intensity modulation ratio in an optical intensity modulator 102 via an amplifier 108.
Then, the above-described measurement is carried out while the output wavelength of a wavelength-variable light source 101 is being swept, thereby obtaining θ(ω), ψ1(ω), and ψ2(ω) from the respective measurement results. Then, the control section 109 obtains the polarization mode dispersion τPMD from the equation (1).
However, according to the method described above, when the output light of the optical fiber under test 104 is split into the s-polarization component and p-polarization component by the polarization beam splitter 105, the power may be deviated. Namely, either the s-polarization component or the p-polarization component output from the polarization beam splitter 105 may be extremely larger than the other component. In an extreme case, only the p-polarization component is output, and the s-polarization component is not output.
In this case, the S/N ratio of the phase of a component with a smaller power degrades, and the measurement of the phase hence becomes inaccurate. Consequently, measured results of a group delay time and the polarization mode dispersion of the optical fiber under test 104 contain a noise, and the measurement thereof hence becomes inaccurate.
A purpose of the present invention is to provide an optical characteristic measuring instrument and the like which prevent adverse influence upon the measurement of the optical characteristics due to a deviation between the polarization components.